The present invention belongs to the technical fields relating to waveform generating circuits generating voltage waveforms, inkjet head driving circuits driving actuators for ink ejection provided on an inkjet head, and inkjet recording devices provided with an inkjet head having actuators that are driven by such an inkjet head driving circuit.
Conventionally, inkjet heads having actuators for ink ejection are well known, and examples of such actuators of inkjet heads are piezoelectric actuators provided with electrodes on both sides of a piezoelectric element, which constitute a portion of a pressure chamber accommodating the ink. When a pulse-shaped voltage is applied to the electrodes of such an actuator, the actuator is deformed such that the volume of the pressure chamber is reduced, thus creating a pressure in the pressure chamber, which ejects ink drops from a nozzle that is in communication with the pressure chamber.
As shown for example in FIG. 8, the voltage waveform applied to the actuators is made of a first waveform P1xe2x80x2 (voltage-falling waveform), at which the potential falls from ground potential to the minimum potential (xe2x88x92Vf), a second waveform P2xe2x80x2 continuing the first waveform P1xe2x80x2 and maintaining this minimum potential, a third waveform P3xe2x80x2 (voltage-rising waveform) continuing the second waveform P2xe2x80x2 and rising from the minimum potential to the maximum potential (Vf), a fourth waveform P4xe2x80x2 continuing the third waveform P3xe2x80x2 and maintaining this maximum potential, and a fifth waveform P5xe2x80x2 (voltage falling waveform) continuing the fourth waveform P4xe2x80x2 and returning from the maximum potential to ground potential. This series of first to fifth waveforms P1xe2x80x2 to P5xe2x80x2 constitutes one driving pulse Pxe2x80x2 for ejecting one ink drop from the nozzle, and the driving pulse Pxe2x80x2 is given out repeatedly with a predetermined period.
An example of a waveform generating circuit (inkjet head driving circuit) generating the voltage waveform (driving pulse Pxe2x80x2) for driving the actuator is shown in FIG. 9. In this drawing, numeral 101 is a CPU, which has two terminals outputting digital signals (for example of 8 bits) for generating the voltage waveform. A first D/A converter 102 for converting a digital signal into a positive analog signal and giving it out and a second D/A converter 103 for converting a digital signal into a negative analog signal and giving it out are connected to the digital signal output terminals of this CPU 101. The first and second D/A converters 102 and 103 receive from the CPU 101 a data set signal together with the digital signals but from a different terminal than the digital signals, and when this data set signal has been input and a predetermined time (data settling time) has elapsed after its input (after the output of the D/A converter 102 (or 103) has settled), the analog voltage is given out. The first D/A converter 102 is connected to a first power source 106 giving out a positive voltage, whereas the second D/A converter 103 is connected to a second power source 107 giving out a negative voltage.
A first and a second voltage/current converter 109 and 110 are respectively connected to the output terminals of the first and the second D/A converter 102 and 103, and these first and second voltage/current converters 109 and 110 convert the positive and the negative analog voltage into currents. The output terminals of the first and second voltage/current converters 109 and 110 are connected to a current/voltage converter amplifier 111, which amplifies the currents into which the voltages have been converted by the first and second voltage/current converters 109 and 110, and converts the amplified currents into a voltage. It should be noted that the first voltage/current converter 109, which is connected to the output terminal of the first D/A converter 102, is connected to the first power source 106, whereas the second voltage/current converter 110, which is connected to the output terminal of the second D/A converter 103, is connected to the second power source 107, and the current/voltage converter amplifier 111 is connected to both the first power source 106 and the second power source 107.
Based on the output voltage from the first and second D/A converters 102 and 103, the first and second voltage/current converters 109 and 110 and the current/voltage converter amplifier 111 generate voltage waveforms like the first to fifth waveforms P1xe2x80x2 to P5xe2x80x2. More specifically, when the first D/A converter 102 outputs a positive analog voltage and the second D/A converter 103 outputs ground potential, the voltage rising waveform (third waveform P3xe2x80x2) is generated, whereas when the second D/A converter 103 outputs a negative analog voltage and the first D/A converter 102 outputs ground potential, the voltage falling waveforms (first and fifth waveforms P1xe2x80x2 and P5xe2x80x2) are generated. Furthermore, when both D/A converters 102 and 103 output ground potential, waveforms maintaining the potential directly before the output of those ground potentials (second and fourth waveforms P2xe2x80x2 and P4xe2x80x2) are generated, and the potential between neighboring driving pulses Pxe2x80x2 is maintained at ground potential.
Then, the generated voltage waveform is applied to a multitude of actuators of the inkjet head, through a current amplifier 113, which is made of two transistors 113a, and a driver IC 114. The driver IC 114 includes for example switching transistors that are provided in accordance with the actuators, and, receiving print signals from the CPU 101, selects the actuators corresponding to the nozzles through which ink drops are to be ejected, thus applying the voltage waveform only to the selected actuators.
However, this conventional waveform generating circuit necessitates two D/A converters 102 and 103 to generate the voltage rising waveform and the voltage falling waveform, and a positive voltage has to be supplied to the first D/A converter 102 and a negative voltage has to be supplied to the second D/A converter 103, thus necessitating two power sources 106 and 107 for respectively outputting a positive and a negative voltage, so that there is the problem that it is expensive and requires relatively much space. Furthermore, discrepancies in the characteristics among the first and second D/A converters 102 and 103 (discrepancies among variation amounts) cause discrepancies among the generated waveforms.
In view of these facts, it is thus an object of the present invention to improve the configuration of the above-described waveform generating circuit, and thus attain a simple configuration that is less expensive and takes up less space, and with which stable voltage waveforms can be attained.
In order to attain these objects, in the present invention, one D/A converter is provided, and when the output voltage of the D/A converter is larger than a predetermined voltage that is midway between a maximum value and a minimum value of that output voltage, one of a voltage rising waveform and a voltage falling waveform is generated, and when the output voltage of the D/A converter is smaller than the predetermined potential, the other waveform is generated.
More specifically, according to a first invention, a waveform generating circuit includes one D/A converter that converts a digital signal into an analog voltage and outputs the analog voltage, and a waveform generating portion into which an output voltage of the D/A converter is input, which generates one of a voltage rising waveform and a voltage falling waveform when the output voltage of the D/A converter is larger than a predetermined potential that is midway between a maximum value and a minimum value of that output voltage, and generates the other waveform when the output voltage of the D/A converter is smaller than the predetermined potential.
With this configuration, a voltage rising waveform and a voltage falling waveform are generated taking a predetermined potential that is midway between a maximum value and a minimum value of the output voltage of one D/A converter as a reference, so that it is not necessary to provide two D/A converters as in conventional circuits, and moreover, one power source outputting either a positive or a negative voltage is sufficient. Furthermore, waveform generation discrepancies due to discrepancies in the characteristics among the two D/A converters, as in conventional circuits, do not occur. As a result, it is possible to achieve a circuit that is less expensive and uses less space, and stable voltage waveforms can be generated.
According to a second invention, the first invention further includes a constant voltage source outputting a constant voltage equal to the predetermined potential, and a switching means, whose input can be switched between the output voltage of the D/A converter and the output voltage of the constant voltage source, and which outputs one of those two output voltages to the waveform generating portion, wherein the switching means is configured such that when a digital signal for generating a voltage rising waveform or a voltage falling waveform with the waveform generating portion is input into the D/A converter, the input into the switching means is switched from the output voltage of the constant voltage source to the output voltage of the D/A converter.
With this configuration, the input into the switching means is set to the output voltage of the constant voltage source when neither the voltage rising waveform nor the voltage falling waveform are generated, and is switched from the output voltage of the constant voltage source to the output voltage of the D/A converter when the voltage rising waveform or the voltage falling waveform is generated. As a result, when neither the voltage rising waveform nor the voltage falling waveform are generated, a predetermined potential is output to the waveform generating portion from the constant voltage source, which can output a precise voltage, even if the D/A converter outputs a voltage that is slightly different from the predetermined potential due to variations of its characteristics, so that malfunctioning of the waveform generating portion due to such variations in the characteristics of the D/A converter can be prevented.
According to a third invention, in the second invention, the switching means is configured such that the input into the switching means is switched from the output voltage of the constant voltage source to the output voltage of the D/A converter after the output of the D/A converter has settled.
That is to say, the time from the input of the data set signal until the output of the D/A converter has settled fluctuates depending on the output voltage of the D/A converter and variations in its characteristics, so that if there is no switching means, or even if there is the switching means, but the input into the switching means is switched to the output voltage of the D/A converter before the output of the D/A converter has settled, then the result is variations in the generation timing (output timing) of the voltage rising waveform or the voltage falling waveform by the waveform generating portion. However, in this invention, the input into the switching means is switched to the output voltage of the D/A converter only after the output of the D/A converter has settled, so that the voltage rising waveform or voltage falling waveform can be generated and output substantially at the same time as the switching of the input into the switching means. As a result, variations in the waveform generating timing brought about by fluctuations in the output settling time of the D/A converter can be prevented.
A fourth invention is an invention of an inkjet head driving circuit driving an actuator for ink ejection provided on an inkjet head, and this invention includes one D/A converter that converts a digital signal into an analog voltage and outputs the analog voltage, and a waveform generating portion into which an output voltage of the D/A converter is input, which generates one of a voltage rising waveform and a voltage falling waveform and outputs it to the actuator when the output voltage of the D/A converter is larger than a predetermined potential that is midway between a maximum value and a minimum value of that output voltage, and which generates the other waveform and outputs it to the actuator when the output voltage of the D/A converter is smaller than the predetermined potential.
With this invention, a similar operational effect as in the first invention can be attained.
According to a fifth invention, the fourth invention further includes a constant voltage source outputting a constant voltage equal to the predetermined potential, and a switching means, whose input can be switched between the output voltage of the D/A converter and the output voltage of the constant voltage source, and which outputs one of those two output voltages to the waveform generating portion, wherein the switching means is configured such that when a digital signal for generating a voltage rising waveform or a voltage falling waveform with the waveform generating portion is input into the D/A converter, the input into the switching means is switched from the output voltage of the constant voltage source to the output voltage of the D/A converter.
Thus, a similar operational effect as in the second invention can be attained.
According to a sixth invention, in the fifth invention, the switching means is configured such that the input into the switching means is switched from the output voltage of the constant voltage source to the output voltage of the D/A converter after the output of the D/A converter has settled.
Thus, a similar operational effect as in the third invention can be attained.
A seventh invention is an invention of an inkjet recording device, and this invention includes:
an inkjet head having a pressure chamber filled with ink, a nozzle linked to the pressure chamber, and an actuator that is caused to eject the ink inside the pressure chamber through the nozzle by application of a voltage;
a relative movement means that moves the inkjet head and a recording medium relatively to one another; and
an inkjet head driving circuit driving the actuator of the inkjet head;
wherein the inkjet head driving circuit comprises one D/A converter that converts a digital signal into an analog voltage and outputs the analog voltage, and a waveform generating portion into which an output voltage of the D/A converter is input, which generates one of a voltage rising waveform and a voltage falling waveform and outputs it to the actuator when the output voltage of the D/A converter is larger than a predetermined potential that is midway between a maximum value and a minimum value of that output voltage, and which generates the other waveform and outputs it to the actuator when the output voltage of the D/A converter is smaller than the predetermined potential; and
wherein recording is performed by ejecting ink from the nozzle of the inkjet head onto the recording medium by outputting to the actuator the voltage waveform generated by the waveform generating portion of the inkjet head driving circuit when the inkjet head is moved in relation to the recording medium by the relative movement means.
With this invention, an inkjet recording device easily can be attained, which is compact, inexpensive and has superior ink ejection performance.